In switched reluctance motors, a power stage provides a constant or variable current to opposite windings of the motor such that a ferromagnetic rotor, e.g. an iron rotor, moves to a position in which the inductance of the excited winding is maximized. A controller stage may calculate the position information of the iron rotor on one hand and provide control signals to the power stage on the other hand, e.g. a control signal triggered by a commutation point which may be determined by the position information. By creating a rotating electro-magnetic field in the stator, a rotation of the rotor may be achieved.
The rotor position information may be determined by a sensor-based method, e.g. from a discrete external sensing unit based on position sensors. This information may alternatively be determined by a sensorless method, e.g. by observing electric and/or magnetic parameters of the motor during motion of the rotor. Sensorless methods are known in the art which use flux integration, observer based methods, fuzzy logic, neural networks and/or induced voltage in order to estimate the mechanical rotor angle. Other known algorithms may rely on inductance based methods, in which the position may be reconstructed through analyzing an induced current waveform, which is a function of rotor angle and current. An example of such an inductance based method is disclosed in U.S. Pat. No. 5,859,518, which describes phase current commutation based on a threshold condition on the observed current.
It is known in the art that the inductance is dependent on the rotor position, but also on the current itself. Sensorless methods based on inductance measurements may be known which handle this problem by complex look up tables. However, such approach may require an initial calibration of the system and/or detailed knowledge of mechanical, electric and/or magnetic parameters of the motor.
Because of the inductance variation, it is also possible to look at the slope of the phase motor current di/dt. If the inductance of the system is maximal, e.g. with rotor alignment in the direction of two opposite energized coils, the resulting slope of the motor current di/dt is minimal.
U.S. Pat. No. 6,586,903 describes a method for determining the position of a moving rotor of a reluctance motor, which comprises sampling the phase current within a conduction period of a phase, e.g. by feeding the current of the motor into an input of an analog to digital (AD) converter. The method further comprises detecting when the phase current has passed its peak, computing when the peak current occurred, and determining the rotor position therefrom. For example, the commutation point may be observed in case two subsequent samples of the AD converter do not deliver any change in current anymore, i.e. when di/dt=0. A specific disadvantage of this method, as mentioned in this prior art document, may be a tendency to react to noise, e.g. detection of zero current slope may be unreliable. Detection of onset of downward slope is suggested as an alternative. The robustness may be further increased by requiring a minimum of consecutive downward slope signals.
Other sensorless methods use the freewheeling currents and thus non energized phases of the switched reluctance drive. A reference is given under U.S. Pat. No. 5,793,179. This method requires three series resistors as additional components in order to monitor the freewheeling current.
U.S. Pat. No. 6,979,974 describes a method for determining the position of a moving rotor of a reluctance motor. This method comprises applying a voltage to the phase winding having a waveform period independent from the period of rotor movement, e.g. applying voltage pulses during motor run; thus these voltage pulses provide current peaks in the motor current. The magnitude of the current peaks is influenced by the linked inductance and thus by the rotor position. This method comprises detecting a feature of the current waveform and deriving a position of the rotor from the occurrence of this feature. This feature may be a change of gradient of the current, for example the point of zero current slope di/dt. However in practice, as acknowledged in U.S. Pat. No. 6,979,974, detection of this point may have limitations and can be unreliable. An advantage of this method is that it will also provide position information for non-energized motors, e.g. during “coasting” of the motor. It is furthermore suggested that the onset of the downward slope is used instead as a more robust feature.